SubD creates a crease in an edge
This can happen when your normals are flipped on part of the mesh.  Pretty common with Mirror operations.
The problem is that the Subdivision Surface Modifier is interpolating between the inside and the outside of the mesh, at this point, so for the vertices in between they are returned to their original positions.

To diagnose problems like this you can check face orientation overlay in this menu.

Typically you want the blue side facing the camera.  If you see any red you can tab into edit mode, select all your mesh and press Alt+N or go to the menu Mesh -&gt; Normals -&gt; Recalculate Outside.